1. Field of the Invention
This invention relates to a process for the removal of fillers from wastepaper and to the use of certain organic phosphoric acid esters for the removal of fillers from wastepaper.
2. Statement of Related Art
Fillers are added to almost all papers to improve their printability, their density and their opacity and to obtain a more uniform degree of light transmission and greater whiteness. The fillers used are mineral by nature or are chemically precipitated products, for example aluminum silicates, such as kaolins or siliceous alumina, calcium carbonates, such as chalk or lime, talcum, calcium sulfate and/or barium sulfate (Ullmanns Encyklopadie der technischen Chemie 17, 577 et seq. (1979)). The filler content of the paper stock depends on the purpose for which the paper is to be used and, in most cases, is between 7 and 25% by weight. To be able to produce a paper having standardized quality features, it is essential that the starting materials and auxiliaries used to make the paper are of uniform quality. Printed wastepaper is used in large quantities in the production of printing paper and tissue paper. To obtain high whiteness, the printing inks have to be removed from printed wastepaper. This is done by deinking processes essentially comprising two steps, namely:
1. pulping the wastepaper, i.e. fiberizing in water in the presence of the chemicals required for detachment of the printing ink particles and
2. removal of the detached printing ink particles from the fiber suspension.
The second step can be carried out by washing or flotation (Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Vol. 17, pages 570-571 (1979)). In flotation, which utilizes the difference in wettability between printing inks and paper fibers, air is forced or drawn through the fiber suspension. Small air bubbles attach themselves to the printing ink particles and form a froth at the surface of the water which is removed by clarifiers.
The deinking of wastepaper is normally carried out at alkaline pH values in the presence of alkali hydroxides, alkali silicates, oxidative bleaches and surfactants at temperatures in the range from 30.degree. to 50.degree. C. Soaps and/or fatty alcohol polyglycol ethers are often used as surfactants which are responsible for the detachment and separation of the printing inks (Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Vol. 17, pages 571-572 (1979)). JP 61/207686, reported in Chem. Abstr. 106, 121694v, describes the use of aliphatic .alpha.-sulfocarboxylic acids and aliphatic .alpha.-sulfocarboxylic acid esters in flotation deinking processes. According to Russian patents SU 773 174, reported in Derwent 51102 D/28 and SU 717 95 reported in Derwent 72992 C/41, good results are obtained in the flotation of printing ink when wastepaper is treated with mixtures containing alkyl sulfonates and soaps. According to U.S. Pat. No. 1,925,372, particularly good deinking results can be obtained when filler-containing wastepaper is treated with aqueous solutions containing soaps and/or sulfonated mineral oils and the paper fibers are subsequently removed by filtration. In the case of filler-free printed wastepaper, fillers are separately added.
Unfortunately, the known processes for separating the detached printing ink particles from the fiber suspensions have serious disadvantages. The high filler component of wastepaper is only removed very incompletely by flotation, so that the proportion of deinked wastepaper is limited to around 50% by weight in paper manufacture, particularly in the manufacture of newsprint paper. Although the fillers present in wastepaper are removed by washing of the paper fibers, there is the disadvantage of a very high fiber loss and very serious water pollution.
It is known from Wochenblatt fur Papierfabrikation 17, 646-649 (1985) that the removal of fillers by flotation can be increased if the wastepaper is treated with aqueous liquors containing alkyl benzenesulfonates in particular as surfactants and not with aqueous liquors containing soaps or nonionic surfactants. In many cases, however, the improvement in filler removal is not sufficient to meet the stringent requirements which the quality of reusable waste paper has to satisfy.